A quantitative comparison of binding to the M1 and M23 isoforms shows that both serum NMO-IgG and most NMO CSF-derived rAbs bind with higher affinity to OAPs than to tetramers and that structural changes in the AQP4 epitope upon array assembly, and not bivalent binding of IgG, drive this increased affinity (10). The gene discussed is AQP4; the disease is neuromyelitis optica.